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What are the advantages of more than 4 propeller blades?
I have heard that propellers with more than four blades are not as efficient as 4 or less blades because of lower thrust which may result from interfering prop-streams. But I see the ATR-42/72 and they have 6 blades. What is the reasoning and the advantages to that prop layout?

There are many ways to define "efficiency" so please give us clarification if possible. Cost per thrust? Thrust per weight? Even noise per weight? For starters, more blades produce more interference with each other (they pass through each other's wake), and more blades tends to incur more maintenance costs.

Related: http://aviation.stackexchange.com/questions/13545/why-dont-aeroplanes-use-propellers-with-more-blades-like-the-intake-on-a-fanjet

You're partially right. More blades are less efficient. But more blades produce HIGHER thrust, not lower. More blades = more thrust with more drag which reduces fuel efficiency. Less blades = less thrust with less drag increasing fuel efficiency. If at all possible, aircraft designers would prefer to increase thrust by using longer blades. But there are limits to how long you can make landing gears. So if you can't go longer, add more blades.

You are right, more blades are bad for efficiency (follow the link for the definition). Ideally, a propeller should have only one blade. Every additional blade will cause disturbances which interfere with the flow on the other blades.
When engine power increases, the propeller disc area should also grow, but this growth is limited by the resulting speed of the blade tips. Once the flow speed there becomes supersonic, the drag at this section of the blade increases without a corresponding increase in thrust. To avoid that, the next best option is to increase the solidity ratio of the propeller, called also the activity ratio. This is done by either increasing blade chord or the number of blades.
Make no mistake, this is bad for efficiency. But if there is enough power available, adding more blades is the best way to transform that engine power into thrust.
Take the Supermarine Spitfire:

The prototype, powered by the 1,030 hp Rolls-Royce PV-12, had a two-bladed propeller
From the Mk II, a three-bladed propeller was fitted to accommodate the increasing power of the RR Merlin (1,470 hp for the Spitfire Mk V).
With the Mk IX, a four-bladed propeller was needed for the 1,575 hp of the supercharged Merlin 61.
From the Mk XII, the more powerful Rolls-Royce Griffon made a five-bladed propeller necessary. Engine power was raised from 1,735 hp to 2,300 hp for the last variant, the Mk XXIV.

Rolls-Royce Griffon-powered Supermarine Spitfire (picture source)
A lower prop speed allows to increase its diameter, but while tip speed will drop by less than the reduction in prop speed (after all, flight speed should not change), the available thrust from this propeller will drop by the square of the speed reduction, since thrust is proportional to the dynamic pressure on the blades. And thrust you get only from the circumferential fraction of the local speed at the blade; flight speed does not count here and does not help to mitigate the reduction. Consequence: You cannot make the propeller bigger and spin it more slowly for a given power.
An extreme example for a propeller with a high activity ratio is the Aerosila SV-27 contra-rotating propeller of the D-27 propfan engine powering the Antonov An-70:

SV-27 propellers on the An-70 (picture source). Eight blades in the forward disc and six in the rear, running at only 1200 RPM. In order to reduce Mach effects, all blades have a swept tip and deep chord.
This can only be topped by something like an ungeared turboprop with a ducted propeller. A turbofan, in other words.

So then, for a contra rotating propeller, do designers accept a loss of efficiency as a acceptable penalty for the other benefits of such a design?

@CarloFelicione No, contra-rotatig props have a higher efficiency because they reduce swirl losses (which are higher with higher power loadings). Their disadvantages are higher mass and complexity, of course.

Quoted: "Make no mistake, this is bad for efficiency. But if there is enough power available, adding more blades is the best way to transform that engine power into thrust." Is this applicable to helicopter's rotor for stationary hover or vertical lifting?

First, we have to define what is efficiency. In case of propeller, we can define efficiency as the ratio of output power (ability of the propeller to produce a given thrust at a given airspeed) and input power (i.e. the shaft power of the engine).
All else held constant, the efficiency the propeller decreases as the number of blades increase due to aerodynamic reasons (like interference). However, if the engine power is increased, the propeller should be able to 'absorb' that; i.e. transmit engine power to the air flow passing through the propeller disk, adding energy and generating thrust.
If the engine power is increased, there are a number of ways to make the propellers absorb it, each with is own share of issues:

Increase angle of attack i.e. blade pitch- The pitch angle is usually set at a value where the aerodynamic efficiency is optimal. Changing it may make the blade inefficient.
Increase the blade diameter The causes two major problems- Longer blades means more tip speed, which increases drag once it reaches transonic speeds; also, the longer blades means longer landing gear for tip clearance or more clearance between two propellers and/or fuselage. This increased structural weight is unacceptable as leads to a vicious cycle. An extreme case of this is the F4U Corsair, which had an inverted gull wing to accommodate the huge three bladed propeller required for the extremely powerful engine.
Increase the rpm The problem is again the tip speed, which can increase drag significantly at high speeds.
Redesign the blade with more camber Again, the blades are usually at their optimal aerodynamic efficiency; increasing the camber may lead to less efficient blades.
Increase the chord This increases the solidity, improving efficiency; however, this leads to increased interference between the blades, which is again bad.
Increase the number of blades This is usually the solution adopted for reasons above. The blades may become inefficient, but the system is able to produce more thrust with minimum impact (like maintenance etc.). As Hartzell says,

...efficiency doesn’t propel the airplane, thrust does

A good example of the effect of the engine power on the number of propeller blades can be seen in the case of Lockheed Martin C130 Hercules.
The initial model, the C-130A used a three bladed propeller to absorb the 4050 shp of Allison T56-9 turboprops.

Source: commons.wikimedia.org
Later models used a four bladed propeller as the engine power was increased to 4590 shp in Allison T56-A-15 turboprops.

Source: planespotters.net
The C-130J Super Hercules uses a 6 bladed propeller to absorb the 4,637 shp from Rolls-Royce AE 2100D3 turboprop.

Source: theaviationgroup.eu
In extreme cases like the Tu-95, contra rotating propellers had to be used to absorb the power from the engines.

Source: airplane-pictures.net

Quoted: "efficiency as the ratio of output power (ability of the propeller to produce a given thrust at a given airspeed) and input power (i.e. the shaft power of the engine)" What is the relation between propeller efficiency and the propeller slip? Propeller slip is defined as the different (delta) between the theoretical of a propeller can travel for every rotation and the actual distance it can reach.


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How do flap track guides work?
I am planning to build a large-scale LEGO airlier with most parts functional. Doing a little looking at pictures and research I see that commercial airliners have an underwing body that houses the flap track guides. How do they work mechanically?

A video of Boeing 737 Flaps extension and retraction ---- A drawing --- The Wikipedia article.

@mins I want to know how the mechanics of it works so I can replicate it with LEGO.

You won't have many freedom to accurately replicate the actual mechanism (there are many different according to the aircraft model). Maybe this will help. Else the drawing (right side) linked above show you the track.

There are different ways of moving flaps. The trivial case where they rotate around a hinge point which lies within the airfoil contour does not increase wing area. Fowler-type, backward-moving flaps are possible in three general ways:

Douglas flap: Here the flap rotates around a single hinge line which lies substantially below the airfoil contour. As the name suggests, this is used on many Douglas Commercial aircraft.

Douglas flap, left: retracted (source); right: extended (source). Their long and narrow supports make them easy to recognize.

Four- or five-bar linkages: These are mechanically more complex, but enable a more compact mechanism. As their name suggests, four or five linkages are connected with bearings and enable a more complex movement. The drawing and the animated GIF below are taken from Wikipedia and show the mobility m which increases with the number of joints. This would be my recommendation for a Lego model; just make sure that the forward bar is longer than the rear bar - this will give you a realistic movement of the flap.

Above: Linkage mobility (picture source). Below: Four bar linkage (picture source)

Sliding block guide: Here you find real rails and rollers which move along them to enable the most complex and precise movement, even for multi-element flaps. While a four-bar linkage will have a continuously increasing ratio of deflection angle to backward shift, a sliding block guide makes it possible to set the deflection angle independent of the backward shift, and allows the biggest backward shift for a given size of the mechanism. See the Boeing 747 flap tracks below for an example.

Boeing 747 flap tracks (picture source). Note that the upper block is supporting the first flap segment and the lower block the second. Actuation is via a spindle; the linkage in the lower left corner is actually the mechanism for moving the flap fairing.

source: Stack Exchange

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Why do wing tip vortices tend to be most dangerous when there is a light quartering tailwind?
I understand how wing tip vortices be effected under a headwind or crosswind. However, what are the differences in effect between tailwind and light quartering tailwind?
Why is it most dangerous when there is a light quartering tailwind?

According to the FAA's Pilot and Air Traffic Controller Guide to Wake Turbulence (section 2.4.5) it's because:

Light: in light winds the vortices stay around longer; if the wind is greater than 5 knots then the vortices move out of the way quickly and break up
Quartering: crosswinds blow the vortices across the aircraft's flight path, which means that if you're landing next then one vortex might still be over the runway. With no crosswind at all, the vortices will dissipate on each side of the runway, out of your way.
Tailwind: the vortices are blown in the same direction as the aircraft, which means that the vortices from a landing aircraft will be blown down the runway rather than away from it. Again, that's an issue for the next aircraft landing.

They have a handy diagram to illustrate the vortex paths from an aircraft at 0, 3 and 6 knots. You can see that a light, 3-knot crosswind causes one vortex to still be in the flightpath after 20 seconds, whereas with 0 or 6 knots of wind the vortex is out of the flightpath completely. You can't see tailwind on this diagram, of course, but the principle is the same: it blows the vortex down the runway (flightpath) rather than away from it.

The wing tip vortices cause significant problems for (small) aircraft taking off after a large aircraft. the FAA Advisory circular 90-23G deals with this.
As an aircraft takes off, the the vortices sink behind the aircraft and move laterally over the ground.

Source: FAA Advisory Circular 90-23G
In case of no crosswind, the vortices simply move laterally away from the runway at low speed.

Source: FAA Advisory Circular 90-23G
Assume that the aircraft is landing after another one. However, if there is a tailwind, the trailing vortices of the preceding aircraft can be moved into the final approach of the landing aircraft.
In case there is a crosswind, the lateral movement of the upwind vortex is decreased while that of the downwind vortex is increased. This means that the upwind vortex remains in the runway longer than it would be.

Source: FAA Advisory Circular 90-23G
Thus a quartering tailwind presents the worst of both worlds: It move the upwind vortex right in the path of the incoming aircraft and keeps it there longer than usual. The situation is similar for takeoffs.

Source: www.gleim.com
This is the reason it is considered to be the most dangerous situation.

Very simply:

A strong tailwind would break up the vortices more quickly.
Quartering, as in not straight on, means the upwind vortex is blown into the path of the next aircraft.

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What is the rationale behind "One engine - Two propellers" configuration?
I stumbled upon a very unusual Russian amateur built ultralight:
One engine, two propellers configuration

My first impression is that it adds another useless layer of complexity and moreover it may involve some safety issue: I particularly think of the entry into a spin in case of one propeller failure.
What is the rationale behind it? The pros and cons?

I think Wilbur and Orville did this first.

It looks as if they rotate in opposite directions. This eliminates prop torque, which should improve handling, especially at low speeds.
On the other hand, you are right with your impression that it adds complexity, but at least it is not completely useless.
Asymmetric prop failure is indeed an issue and will markedly reduce handling qualities, and for that reason both props are close together, so if one is left, the yawing moment should be manageable if the craft flies fast enough.
Putting a single prop in the center would present it the messy flow coming from the unfaired cockpit and engine, so placing them left and right should improve inflow conditions. They are still close enough so the tail is in the prop wake and will have improved effectiveness at slow speed and on the ground. On the other hand, the outriggers will create additional drag, and it is hard to say if this arrangement will have performance benefits.

Does the number of propeller have something to do with efficiency too?

Also: 2 small pusher props rather than 1 large pusher = (1) less vertical space needed in airframe, (2) props in cleaner air?

To me it looks like they are driven off the same pulley and neither of the belts crosses between the motor and the prop, So to me it looks like they rotate in the same direction.

@ratchetfreak: Yes, when looking again I am not so sure anymore. But driving them in the same direction would give away the main benefit of this arrangement.

There's also the point that a propellor is hugely unlikely to fail compared to an engine failure in a two-engine-two-prop design

Basically it allows the benefits of multiple propellers with a single engine. At low powers increasing the number of engines decreases power-to-mass ratio. Additionally, the builder might simply have had only one engine available or affordable.
Removal of prop torque, as mentioned in the accepted answer, is a clear benefit. Probably relevant for this particular ultralight, having larger propeller area at limited height. Generally having larger propeller area for power improves efficiency and reduces noise. In the design in the image the height is especially limited. Also not having the propellers in the same line with the body probably improves air flow slightly.
I think the limited height available in the design is the actual main reason.
It is slightly more mechanically complex than single prop, which increases weight and chance of failure.

From the title I thought we'd be looking at pictures comparing a Corsair and something with contras; height requirements. The duel props on this one strike me as the way to get more torque out of the engine.

@Mazura Good point. I already mentioned having only one engine available as possible reason, designers engine choices might have been limited in more ways than just the number of engines.

While the currently accepted answer would be valid in terms of eliminating prop torque had that been the setup, a close look at the photograph shows both propellers rotating in the same direction (counterclockwise from the point of view of the viewer) - the pitch of each blade is clearly visible.
I would suggest that in this application, the two propellers are used to give the benefit of an equivalent larger overall propeller (greater propeller area), without the additional ground clearance or vertical space requirements - a larger propeller puts through more power for the same engine RPM.


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Does a prone position for the pilot minimize g-force effects?
Knowing that the Wright Flyer was the first successful heavier-than-air powered aircraft, I'm fascinated by its operation requiring a prone position.
Searching further, I was surprised to find out that the recent EMG-5 motorglider also puts the pilot in a "prone" position.

My question:
Can someone expand on the alleged minimizing g-force effect of the prone pilot position. Why isn't it more common, especially in fighters?

It looks pretty uncomfortable to me - imagine holding your head & helmet like that for 30+ of aerobatics.  Ouch!

That's why I tagged the post [tag:human-factors] :-)

It is true that the body can withstand higher g-forces in the prone position. The reason is that in the prone position, g-forces don't pull blood down to the lower body and away from the brain, as happens in the seated position.
The concept was explored for combat aircraft with the Italian Savoia-Marchetti SM.93 and the British Gloster Meteor F8. These aircraft revealed some practical issues with the prone position:

It is much less comfortable for the pilot than a seated position, especially during long flights.
It is much more difficult to design an ejection/escape system. The Meteor F8 required the pilot to jettison the rudder pedals and crawl backwards to an escape hatch.
It affords much less visibility to the pilot.

In light of these issues, it was found more practical to stick with seated pilots. In modern times, g-suits and advanced training techniques like the "hook" maneuver have allowed fighter pilots to withstand higher g-forces than in the past. Both involve reducing blood pooling in the lower body by squeezing or flexing leg and abdomen muscles to restrict unwanted blood flow and pooling.

Escape is not a problem: The DFS 228 used a jettisonable cockpit section which would descend on its own, small chute, and when the cockpit was low enough, the pilot was extracted backwards by his own parachute.

@PeterKämpf ... which was probably a lot harder to design and more expensive than a traditional ejection seat.

… not when the typical operating altitude is 60.000 ft. The DFS 228 was planned to be launched in piggyback position by another aircraft and to be powered by the same rocket engine as the Me-163 for flight far above any other aircraft. I would also dispute that this would have been harder to design than an ejection seat - at that time (1943) you couldn't get them that easily.

The prone position was researched a lot in Germany before anti-g suits were invented. The human body can tolerate g loads a lot better lying than sitting. Martin Eiband collected a lot of data on this, if you want to know more, google for "Eiband diagrams". If you want the full picture, read the Army Aircraft Crash Survival Guide. It comes in five volumes, and volume 1 covers the design criteria. The Eiband diagrams below are lifted from this source.

The frontal acceleration is quite similar to that of the lying pilot. Please note that for a duration of 0.1 seconds, the limit is 40 g! For 1 full second, this goes down to 10 g. Any more, and the pilot might show mild symptoms of stress. And if you go up to approx. 23 g for 1 second (logarithmic scale!), there is a risk of serious injuries.

This diagram shows the limits for a sitting human. At 0.1 seconds, the limit is only 10 g. For a full second, this goes down to 5 g, even though the data does not cover this duration.
One notable research plane was the B-9, built by Akaflieg Berlin, a student group which designed and built airplanes during their time at Berlin's Technical University. This airplane was designed for 22 g and used a prone position for the single pilot. It was tested in 1943 and proved the validity of the concept. Previously, Akaflieg Stuttgart had built the fs-17 for the same purpose. Also, the Horten flying wing gliders were flown in a kneeling-prone position.

Horten IV pilot position. Note the chin rest!
There was even a ground attack aircraft designed for a prone position, the Henschel 132, a small, jet-powered single seater. A prototype was almost finished before the war ended, and it never was used operationally. Similarly, a rocket-powered, glider-like high altitude reconnaissance plane, the DFS-228, was only flown without the rocket engine.
The prone position has a major disadvantage: Visibility is poor where it counts most. Downward visibility is great, and even forward visibility can be considered acceptable, but especially in turns the pilot must be able to look up. A tight turn is mostly a horizontal looping, and a lying pilot needs to lift his head up against the g forces in a turn - this is extremely tiring! The whole concept is impossible for air combat, where g forces are highest.
The prone position has an advantage for small aircraft where the drag of a sitting pilot would be unacceptable. It has only found widespread use in hang gliding, but all other aircraft prefer a sitting or, mostly in gliders, a backwards lying pilot.

The chin rest make me think of violin :-)


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Why does enriching the mixture ratio beyond stoichiometry give more power?
In other words, in a piston engine, the best-power mixture is richer than that producing the EGT peak that signals stoichiometric combustion.
Why is that, physically?

Welcome to Av.SE!

Is it actually true that peak EGT corresponds to a stoichiometric ratio? Bear in mind the indicated EGT depends on the speed of combustion as well as the chemical energy released, since it's sensed at a particular point in the tailpipe.

Related, on space.SE: Pro/cons of burning propellant in stochiometric ratio?

It's because, although the Best Power mixture dumps more fuel than can be consumed by the available oxygen, the extra fuel increases the combustion speed in the charge, increasing the buildup up of pressure in the combustion chamber, and this increases the total potential energy in the air/fuel charge that can be converted to work, even though not all of fuel is burned.  You could say that the excess fuel has kind of a catalytic effect, or maybe a turbocharging effect... sort of... up to a point.
Beyond a certain point, adding more excess fuel becomes counterproductive, so Best Power mixture is the sweet spot where the speed of combustion and pressures generated are maximized and the cylinder makes the most horsepower it can make (the highest possible combustion pressure) with the available displacement and airflow.

I don't know if by turbocharging effect you are referring this but: evaporation of the fuel cools down the air charge on its way to the cylinder. More fuel, cooler air charge, more air going to the cylinder. Up to a certain point (max power) this might be of significance. Might, because I don't remember and can't find any sources about this to confirm if the difference is significant between max power and stoich in gasoline engines. When comparing gasoline and ethanol engines, the difference is significant.

By difference you mean the amount of evaporative temperature drop?  To the extent it does, I would think it's only a factor in carbureted engines where the droplet stream has a long way to go.

Yes. That might very well be the case. Difference in the amount of gasoline between stoich and max power is not that big after all.

I am pretty sure the rich mixture "decreases" combustion speed (increases RON), allowing spark advance and favouring any forced induction boost. Water can do the same. Takeoff power was traditionally hard on pistons, which are really only ring carriers. The heat and stress is considerably higher. A rich mixture is less "knocky" as the combustion speed slows.

in practical terms, running rich means that despite the unevenness of the fuel-air mix both in a single cylinder firing and across all cylinders firing and across all firings in the engine, all cylinders are guaranteed to NOT be running lean on ANY firing, which means all of them will be pulling best power on EVERY firing.
In addition,  there is an effect that I know about from motorcycle engines which is probably true of airplane engines and that is the "quality" of combustion as a function of richness: If you run rich, preignition and lean misfire (both of which reduce combustion efficiency) are prevented, and the delivered power is smoother. You can hear this: a motorcycle engine sounds "happier" when running rich than it does running lean- and it pulls harder, even though the mix is nonstochiometric.

This is by far a more correct answer, in my opinion.  Because the fuel and air is mixed at the carburetor, not all cylinders get the same amount of fuel.  The front valves get more fuel than the rear valves do.  In a fuel injected engine, the fuel is introduced just before the cylinders and each gets approximately the same amount of fuel.  On fuel injected engines, with individual cylinder head temperatures it is possible to run lean of peak and get some pretty incredible performance.

@wbeard52- agree, and especially if the fuel injectors have been individually tuned to behave exactly the same. there is an (expensive) way to accomplish this which is reputed to furnish outstanding performance at several GPH less than standard for the power settings used.

This is simply due to the difference between theory and practice. It turns out that even in a well-balanced charge of fuel and air, there are highly localized “pockets” of varying mixtures at the local, molecular level. By “local level” you should think of a bunch of little fuel molecules huddling together over here and over there in different places inside the cylinder as the piston is rising up towards top dead center and starting down. Some of these “pockets” may be so lean, or so rich, that they won’t burn at all. Some may be in the combustible range, but not really contribute to the combustion event, and some may be perfectly mixed, ready to go, so to speak.
In theory, the ideal mixture for our engines is about 15 parts air and 1 part fuel (by weight), which should result in no oxygen and no unburned fuel molecules going out the exhaust pipe - IF it was a truly homogenous mixture, but, it’s not. It’s actually highly dynamic, varying greatly from cycle to cycle. Each combustion event is comprised of those little pockets of varying fuel-air mixtures I just described, different every time. At a stoichiometric mixture, after the spark occurs and the flame front propagates, some of the oxygen molecules just do not “find” fuel molecules quickly enough to burn, and they remain unused or unburned at the ideal ratio. By supplying just a bit more fuel, there is a greater opportunity for the lonely oxygen molecules to react with the hydrocarbons, meaning more total fuel is burned, a bit more heat is generated, and less oxygen escapes out the exhaust pipe without having had a chance to mate.
This is why we’ve long observed that a slightly richer mixture would produce slightly more power.

Like the answer! In the 2nd paragraph, last 2 sentences, why do extra fuel molecules for the O2 to find, work better than extra O2 molecules for the fuel to find? IOW, why wouldn't swapping "fuel" with "oxygen" in those 2 sentences be an equally accurate explanation, since there unburned molecules of each "hanging around" as described.

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text:
For how long after the last A380 delivery does Airbus have to provide spare parts for it?
After Airbus's announcement that they plan to discontinue A380 production in 2021 after the last orders are filled, how long will they have to guarantee the availability of A380 spare parts?

I don’t think the last delivery is important, all that matters a contracts and how many customers are willing to pay for spare parts. A passenger aircraft should fly for around 30 years. So this is a least a hint. Actually maintenance and the number of pressurization cycles matter. If you fly low enough, only spare parts matter. The DC3 or JU52 still fly. I think the B52s are flown seldom, so they are still in use after 50 years. Let‘s order an A380 and see what Airbus offers ;)

When a company decides to terminate the construction of a specific airplane, then this company needs to follow some conditions. In the contracts between Airbus and airline companies, there are many conditions concerning multiple aspects.
There are conditions for example about:

spare-parts availability
transfer knowledge about specific features of the airplane
training (not for all the occasions)
and more (we are not in a position to know them all and in detail)

These conditions are discussed in the beginning, before the signing of the contract. Of course, conditions may differ from one contract to the other, but I think you got the point behind this idea.
So there is no way that someone will tell you what will happen between Airbus and their clients because this information is confidential. As soon as, the clients of Airbus do not sue them, then their contracts are including mechanisms about this.
Lastly, these contracts most of the time are including basically conditions about safety or financial sections. There is coming the role of ICAO, IATA, EASA, FAA and more. These organizations are responsible for managing properly also the sector of safety/security. The fact that spare-parts availability is playing vital reason in the safety of the passengers among the world, then it is 100% sure that they also know about it and they know how to handle it.
Hope that I helped.

What, exactly, are the regulations you are talking about? As is this answer is vague.

@BSteinhurst I am trying to say by my answer, that the contracts between airline companies and Airbus are having regulations. These regulations include the concerns around the spare-part availability. Additionally, I wrote such a huge answer to include future questions concerning this kind of topic. Today is airbus with the A380, tomorrow is Boeing or Embraer. It's a generic answer and at the same time speciliazed too.

I think the word "regulations" might be confusing here because it makes people think about the legal regulations from the FAA, EASA etc. You seem to be referring to "requirements" or "conditions", which can be different in every contract.

There is an aircraft company in our country, Indonesia. When that company sold one of the product (CN295) to Thailand in 1996, there is mentioned by commentator on tv, that one of the term and condition is that the company has to guaranty the sparepart availability for 50 years. Otherwise, the manufacturer will be sued and pay pinalty. Thats one most critical point in aircraft manufacture that it have to keep provide it component for so long. I believe, the same thing also applied to AirBus. Otherwise, the super jumbo will be not usefull after fly some hours.

@Pondlife you are right, I will change the word. I will make it conditions. Thank you.

@Aircraft Lover exactly! That's another example of what I said. This happened in many countries and for many companies. It's the most common way of signing a contract.

In the UK the A380 will only be able to operate on a Certificate of Airworthiness, and one of the conditions for the CAA issuing that is the support of the manufacturer.  There are a number of 1930s and 1940s DeHavilland aircraft operating on CoAs, I guess BAE Systems must have picked up that obligation?

@DaveGremlin I am not sure about the example you just mentioned. But I pretty sure that there is a condition which enables the usage of these airplanes. If not, then the company may have troubles with the law or with some regulations with organizations concerning aviation.
It's also well-known that the public sector can also guarantee for some operations. I don't know much about the UK though. Thank you for your information, I am going to search it more ;)

@avionerman - I'm not sure that a public sector aircraft would qualify as a 'State Aircraft' under the Chicago Convention, here's the EASA take on that:  https://www.eurocontrol.int/sites/default/files/article/content/documents/single-sky/cm/library/cmac-definition-of-state-aircraft-200107.pdf You can fly an aircraft on permits other than a CoA in the UK, but I think it would be difficult to get it covered by your Air Operator's Certificate  https://en.wikipedia.org/wiki/Air_operator's_certificate

Many times there exist public sector's companies which produce airplanes, trains, buses or more. These public companies are having a contract between a country and a company. That means that also the country itself needs to guarantee the support of these products. That's what I wanted to say as an example.

Yes, there may be a time lapse during which airplane makers have a regulatory committment to provide spare parts, a Turboprop Canard having few sales forced the builder to start purchasing all remaining airplanes, it was cheaper to them than fulfilling the compromise to provide parts

A380 is a-hundreds million dollar price. So, there should be any guaranty from Airbus that they will provide the sparepart for the buyer. Otherwise, when the production is ceased, how the buyer will replace their faulty component?

Perhaps not needed, if the spread of 3D Metal Printing, Additive Metal Manufacturing, Powder Metallurgy, allows airplane owners to produce the spare parts they need, this will ease problems for those having purchased a monster airplane, that turned 90º another plane on airport, with the wingtip, and sent to wreckage a business jet entering A-380 wingtip vortex. The Convair XC-99 case, rejected by airlines because of the load a sudden huge number of passengers would have imposed on airport facilities sounds similar. Salut +.

Very interesting. If a manufacture fabricate a component of a such as expensive airplane, there should be FAA and other authority's certificate, too, right? As in my understanding, certification is a long process, and very not easy. Would be another manufacturer allowed to provide the component without certification?

@Urquiola "Perhaps not needed, if the spread of 3D Metal Printing, Additive Metal Manufacturing, Powder Metallurgy, allows airplane owners to produce the spare parts they need"* This is not correct, at least not for safety-critical parts. These parts are qualified including their manufacture method, so replacing e.g. a cast aluminium part with a sintered or 3D printed one without the designer's approval would render the airframe not airworthy.

I guess there is a lot of evidence Metal 3D printed parts have same or higher quality than former cast and machined parts, so say airplane builders, who are implementing this method. About Aviation Authority Certification for parts, well, the decals to put on wings, indicating: 'No Step' were considered 'critical parts', to be purchased from airplane builder at prices over $100. Everybody had this warning hand painted. The part that fell down from a previous airliner on same takeoff strip, cause of Concorde destruction, was a 'non certified' pirate part!

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Are electromechanical actuators used in aircraft other than the Boeing 787?
Are there any large (non-experimental) civil or military aircraft that use electromechanical actuators (EMA), other than the Boeing 787?

Are you asking about the particular type of advanced fully integrated electric actuators used on the 787, or electric linear and rotary actuators in general?

Electromechanical Actuators are used in many aircraft, and in many applications.
Applications vary from controlling doors,  trims, control surfaces, flaps and air inlets to moving your business class seat to the most comfortable position.
For quite a range of examples see the product website of this manufacturer.
Or this video of an aileron application


text:
How can I switch off the yaw damper in an Airbus A350?
How can I switch off the yaw damper in an Airbus A350?
In the A320, this can be done by switching off the FAC, but the A350 doesn't have a FAC switch; it has prim and sec instead of FAC and ELAC.

I'm curious why you are asking.  Would you mind sharing your interest in the A350, and why you would want to know something so specific?

I'm an engineer at a flight simulator company and I've just been put on the A350 project but I'm not familiar with the cockpit layout.

As a simulator engineer, shouldn’t you somehow be able to receive documents from aircraft OEM detailing the systems to be simulated?

If you want to buy a simulator you pay for the data package but it is the simulator manufacturer who will get it

@MichaelHall This is a Q&A site that frequently receives specific questions about an aircraft's feature or behavior. You don't need to play security and find out why folks are asking a specific question.

@zymhan, who says I’m playing security?  I thought it was a very odd question in the specificity it was seeking.  Most questions from the general public are more like how far, how high, how fast, etc.  And if you really needed to know that kind of detail, as Cpt Reynolds points out, you ought to have access to manuals.  The OP satisfied my curiosity, and you don’t need to scold me for asking.

On the simulator normally you can simulate the failure of the yaw damper actuators which has the same effect as switching off the yaw damper.
On the aircraft you can’t do it from the cockpit panels without switching off lot of other functions

The thing is there are no yaw damper actuators on new A330s, A345 and above. They have all-FBW rudders.

As far as I remember on Airbus in addition to the ADIRUs you have lateral  specific accelerometers to the FBW; you have to switch off all these by means of push buttons and breakers, but this is not enough because in Airbus you have an emergency autonomous yaw damper unit that uses hydraulic power to generate electrical power to provide yaw damper.

If you prevent the yaw damper function to be calculated it will switch off. As yaw damper is calculated using lateral data from ADIRUs and specific FBW lateral accelerometers you need to switch off all these; the ADIRUs by push buttons and the lateral accelerometers by specific breakers. On some Airbus you still have an emergency AUTONOMOUS yaw damper unit, some models are fully electrical and use up to the battery power, so you pull the relevant breaker, but some models use the hydraulic power to generate their own electricity so you need to switch off the corresponding hydraulics, leaving active the only hydraulic that the autonomous yaw damper does not use.
On the other hand note in direct FBW mode the yaw damper is degraded but still functional, so being in direct mode is not sufficient to inhibit the yaw damper. But by putting off the yaw damper most probably you will be in direct law.


text:
What is the function of cross bracing of interplane strut?

Pic 1 is the top and lower wing of a biplane called Bloop Ultralight,what is the function of the cross bracing between the interplane struts  as seen in pic 1(I'm not talking about landing and flying wire)?
Is it there to resist wing twist?  If yes, why is it not in between the interplane struts closer to the wing tip since wing twist and torsion is greatest at the wing tip?....as seen in pic 2

The cross bracing is to prevent the upper wing collapsing onto the lower one, the Bloop is an ultralight without the fuselage that the wings are normally attached to.
The two wing struts are in parallel and together with the parallel wings form a shape which must be cross-linked.. The bracing is for defining and retaining the structure and shape of the wings relative to each other, not to compensate for wing twist.
In the OP picture I cannot see any evidence of wing twist, only a constant wing incidence. The bracing wires need to be inboard only where normally the fuselage provides shape definition: in a classic biplane, there is no cross wiring required at the outer struts.

The bracing wires prevent their surrounding rectangle from "racking" into a quadrilateral.  The forces that would cause that are each wing's pitching moment (that may be what you mean by wing twist, rather than washout).  But why the wires are inboard rather than outboard, I'm not sure.  Inboard, they reduce the airplane's roll inertia slightly.  As this is a slow ultralight, the pitching moment may be gentle enough that inboard and outboard bracing isn't needed.


text:
Is there an additive that can lower the freezing point of jet fuel?
Does an additive exist to decrease the freezing point of jet fuel? I have found an additive that decrease the freezing point generated by drops of water (appears between 0 and -10°C), but I'm needing to reduce the freezing of the fuel (-45°C).

Jet A freeze at -40 C. Jet A-1 freeze at -47 C. Maybe just switch to jet A-1?  https://en.wikipedia.org/wiki/Jet_fuel

Isn't stuff called Prist used to prevent freezing?  https://en.wikipedia.org/wiki/Fuel_system_icing_inhibitor

You may want to elaborate a bit on your use case to help us out. Jet fuel already has a pretty low freezing point and airplanes have no problem keeping it liquid throughout a flight. Spacecraft generally solve the issue with tank insulation which may be of interest to you.

Concur with @vasin1987, Switch to Jet A-1 and problem solved.

There is an additive known as a Fuel System Icing Inhibitor (FSII), it is designed to be mixed with the fuel of non-fuel heated aircraft i.e military. Mixed in at about 0.10-0.15% is optimal to reduce the formation of ice crystals, which will stop it freezing.
Source: http://www.advancepetro.com/fsii.htm
Alternatively, Jet A-1 fuel has a freezing point of -47°C, which is below your threshold and could be useful. Or if Jet A-1 is unavailable you could try Jet B which has a lower freezing point of -60°C, but is more dangerous to handle, and is less prevalent and hence unavailable at many locations.
Sources: http://webarchive.nationalarchives.gov.uk/20100814170713/http%3A//www.dstan.mod.uk/standards/defstans/91/091/00000600.pdf
http://www.csgnetwork.com/jetfuel.html

Welcome to Av.SE!

anti icing is clearly a different thing. more to prevent the water content from icing, not prevent the fuel content from freezing.

It was also mentioned somewhere around here that some newer engines are not allowed to be used with Jet-B.

Welcome to Av.SE. There is also JP-4 that can go down to -57°C. JP-8 is Jet A-1 plus more additives that may have better availability in the US than Jet A-1.

Question to Wehrner von Braun: "Why did you name it Saturn?". Answer: "It was the one after Jupiter!"
The lower freezing jet fuel is Jet B.
Jet fuel (kerosene) is taken from a higher boiling fraction than "naphtha" (gasoline) when refining crude oil.  A mixture of kerosene and naphtha will lower the freezing point.  This is how Jet B is made.
Before doing this in your garage, make sure there is no water in the mix, as frozen ice crystals will be present at much higher temperatures.

Also, don't do this in your garage, as Jet B burns/explodes much more enthusiastically than Jet A-1, and at much lower temperatures.


text:
What aircraft looks like a cross between a Cessna and a glider?
I always like to look at flying machines overhead.  One day I saw something I've never seen before.  It looked like a cross between a Cessna and a glider.  It was powered, but I heard no sound from it (but I might not have anyway due to ambient noise).  The wings were about twice as long and half as wide as a regular Cessna.  Any idea what that was?  It looked like it was built for pure fun.
Updates in response to comments:
It was not a glider; it had a front engine and high wing like a Cessna.  But it was not a Cessna either.  The wings were about twice as long as a Cessna, and about half as wide.  On the other hand, the wings were about half as long as a glider, and twice as wide.
It was almost directly overhead, about 1,000 feet.  I looked intently since I love aviation in all forms and I'd never seen anything like it.
Whatever it was, it looked like it would be a lot of fun.
If it matters, it was near downtown Seattle about 2 years ago.

Maybe something like this aircraft? It is difficult to know what you are talking about from the description you provided...

I've never seen a glider that could be in any way confused for a Cessna (any Cessna), even with the different wing size you describe. More likely you saw a normal aircraft and it was just too far away to clearly hear the engine (and/or the engine sound was masked by something else).

There are a few high wing gliders about. There're also some high wing ultralight/microlights

The Pipistrel Sinus [Flex] is a high wing motor glider with tricycle gear option.

As you describe it, a motorglider would fit the bill. There are several types around, the most popular being the Grob 109 and the Hoffman Dimona.

Grob G-109 (picture source)

[Hoffmann H36 Dimona (picture source)
Other possibilities include the Valentin Taifun (the only one with a tricycle landing gear) or the Scheibe SF 36. All of those are of composite construction, but there are also earlier types with wooden wings and a steel tube fuselage, like the Scheibe SF 25 "Falke".
While a Cessna is a high wing plane, all motor gliders are low wing (well, except for this one here). Would a cross between a Piper and a glider describe what you saw even better?

It may have been a motor glider which is effectively a glider with a small power plant often to get it off the ground under its own power or just to help maneuver and climb when needed. The power plans is generally shutdown in flight and the aircraft is flown like a glider.


text:
What are the customs decal and eAPIS requirements for flying to the Bahamas?
I'm planning to rent an aircraft and fly to the Bahamas. I've got all the requirements including the Restricted Radio license.
However, there was another requirement which concerns paying of the Annual decal fee through CBP DTOPs and filling the form through eAPIS which kind of got me confused. Now I'm about to pay the annual user decal fee for a private aircraft but is this required if its a rental aircraft?
And could someone please briefly explain the difference between the two and how they should be used for my trip?

Have you tried calling Customs and asking? Seems to me the requirement is on you to obtain it as you are taking the plane out of the country and want to be able to come back in with minimal hassle.  Don't forget to check in with Both Immigrations and with Customs when you get to the Bahamas so you have the correct forms on hand when you leave.

Ask the company you're renting the plane from.  They may already have the decal.

AOPA has a useful guide to flying to/in the Caribbean, including the Bahamas. To answer your specific questions:

You do need a customs decal if your aircraft is considered private by the CBP, regardless of whether you own it or rent it (I've seen them myself on several aircraft that I've rented)
eAPIS is a system for reporting who is on board an aircraft every time it crosses the US border, it has nothing to do with the decal


text:
Is it possible for a ramjet to start from 0 velocity?
I was wondering if it is possible for a ramjet to start from 0 velocity.
I know its working principle and all, but is there any missile or aircraft that completely uses ramjet, with no rotating part to compress the air?

Maybe if you drop it from orbit :)

I have no intention like that :)

0 velocity relative to what?

Relative to the ground, I know it can be used after carrying aircraft reaches sonic.

No, a ramjet can't be started at zero speed.  Yes, there are missiles that use ramjet propulsion without any rotating parts: they use a rocket to accelerate the missile to high enough speed for the ramjet to operate and develop useful thrust.
More or less by definition, a ramjet requires significant forward velocity to compress the intake air sufficiently to produce useful thrust.  Depending on the design, this speed might be as low as about 250 kt (120+ m/s) or somewhat above local Mach 1.  That means something has to push the engine (and attached aircraft) to that startup speed.
That "something" can be a turbojet or turbofan engine, a rocket engine, a catapult (I'm not aware of any ramjets that are launched this way, but it's technically possible) -- even a literal gun (ramjet boosted rounds in approximately shotgun sizes were demonstrated as long ago as the 1970s -- NOT the same as a Gyrojet, which was a tiny rocket-only projectile).  For missiles launched from a jet warplane, it might be possible to use the plane's forward velocity, or a boost from a gravity drop, to get this initial speed.

@Michael Hall edit? :P

Edited as suggested by @MichaelHall

Shotgun sizes? Do you have a reference? All I can find are artillery rounds.

Seek and thou shalt find the Supersonic Trebuchet.

@IconDaemon I was thinking of that when I wrote that sentence, though in practice a steam catapult or similar could pretty readily get a ramjet machine up to 400+ kt, fast enough for some ramjets to produce useful thrust.

No. A ramjet relies on stagnation pressure at the inlet in order to compress air prior to combustion.  They are typically not effective until the speeds get quite high, somewhere in the Mach 2+ range.
Ramjet powered missiles which used rocket boosters for launch have been around for decades, possibly the most famous one is the US Navy’s TALOS missile.  One of the most ambitious projects from the Cold War was a weapon called Supersonic Low Altitude Missile (SLAM).  This would have used a nuclear powered ramjet engine and roamed the skies at supersonic speeds for months on end with a nuclear warhead, waiting for instructions to attack enemy targets.

Wow, I was unaware of that project. It'd be the nuclear war analogue of having hot water circulating in your pipes all the time (through a more complicated plumbing system, of course), so that you'd when you wanted hot water from a faucet you wouldn't have to wait. :)  I guess some people have had that hot water set-up... :)

@paulgarrett SAC is already sort of that.

The OP clarified in a comment that they are talking about ground speed not air speed.
It is trivially possible to start a ramjet at zero ground speed, since ground speed is completely irrelevant. The only thing that matters is air speed. All you need is high enough winds.
You could put the ramjet into a wind tunnel (hypersonic wind tunnels can generate winds of Mach 5+, there are even wind tunnels which can reach Mach 27 for about a second). A Category 5 Hurricane would also work, as would a strong tornado.

Thanks a lot for the answer.

In theory you could have an engine that started in pulsejet operation and switched over to continuous combustion as the speed increased.
In practice, good luck with that. Pulsejets require very specific geometries and a fair bit of black magic; ramjets require very specific geometries and a fair bit of black magic. Trying to build something that works in both modes (much less if you want a transition that isn't "turn off the pulsejet, reconfigure to ramjet mode, dive, and hope the ramjet ignites before you run out of sky")... again, good luck with that.
(...and if you want 'no moving parts', you're asking for a valveless pulsejet, which adds a whole 'nether layer of black magic.)

And yet, Colin Furze built a Lockwood type with nothing but a welder, pressure washer, and angle grinder.  The "black magic" here is that valveless pulsejets always seem to have the intake pointing backward, which doesn't work well for a ramjet.

@ZeissIkon - the problem isn't "make lots of fire and non-zero thrust with silly amounts of fuel at zero speed at sea level with unlimited weight and space". It's "make a decent amount of thrust with survivably low fuel consumption over a range of altitudes and speeds in a reasonable weight and volume and not shaking the craft to bits in the process" where the black magic comes in.

@ZeissIkon - the intake might be less of a problem than you might think. Look at the Messerschmitt nozzle.

That "not shaking the craft to pieces" part just isn't happening with ordinary pulsejets.  Maybe with a Gluhareff type pressure jet (they still pulse, just not as hard).  Pulsejets usually trade off efficiency for simplicity -- a jet engine with no moving parts and literally one piece may be worth burning some extra fuel, and some valveless pulsejets can produce above 1:1 thrust/weight including a short-burn fuel tank.


text:
What is the average aerodynamic load on a control surface of a commuter-sized airplane?
I'm doing a study about aircraft hydraulic pump sizing. In order to do that, I need to know the size of a flight control actuator, then determine the maximum flow and pressure that the actuator needs.
Therefore, I need to know how much force that the flight control surface needs to be moved.
What is the average aerodynamic load on a flight control surface of a commuter-sized airplane (10-20 passengers)?

If you’re not too much into in aerodynamic calculations and researching associated literature (I’m not really sure where best to start there...), another approach might be to find which actuators were used in real aircraft and make conclusions from that.

I think you don't actually want to know the load but rather the hinge moment of control surfaces. The actuator load is the hinge moment divided by the length of the control horn. Below is a rather poor sketch for a typical aileron linkage, but the principle is correct (source):

It already shows one popular way of reducing control forces: The tab, a little auxiliary control surface which moves against the "real" control surface. This reduces effectivity a bit, but forces by a lot. Here, the amount of deflection of the tab is controlled by a spring in its linkage, which is a clever way to adjust its deflection such that the actuation forces become more constant over speed.
Another way of reducing control forces is a horn: An extension of the surface forward of its hinge line, so the aerodynamic loads here balance those on the surface behind the hinge line. The picture below shows the left aileron of the ATR-72 which is moved by mechanical linkage (source)

This way, the lift loads on the control surface are mostly carried by the hinge and only the actuation loads need to be carried by the control rod or actuator. If you think you don't need all those nifty tricks, your actuator and hydraulic system will become much heavier than needed.
Why are two different methods used? The tab reduces the loads from deflection changes while the horn reduces those from angle of attack changes, too. When sized properly, both together will drive the hinge moment close to zero.
Why do I explain all this? It shows that your question does not have a simple answer. Rather, you need to specify exactly how your control surface looks and is moved, and only then can you start to calculate the actuator loads. I also want to show that a subsonic airplane for 10 - 20 passengers will be perfectly flyable with manual controls. The ATR-72 needs hydraulics only for the flaps, the spoilers, the brakes and the landing gear. Avoiding hydraulics for primary flight controls also lets it get away with single redundancy in its hydraulics system.

As you noted, there are lots of complexities to be considered when sizing control actuators, including the size of the control surface, the desired deflection angle, the actual deflection angle, hinge moments, boundary layer effects etc. You can obtain a rough approximation of force on the surface though starting with the simple definition
$$
P = \frac{F}{A}
$$
Where P is the dynamic pressure, q, and A is the exposed control surface area. Solving for force gives:
$$
F = PA
$$
Then substituting dynamic pressure and exposed area results in:
$$
F = \frac{1}{2} \rho V^2A sin\delta
$$
Where A is the control surface area, and multiplying it by the sine of the deflection angle yields exposed area.
For a light transport aircraft, say a Beech 1900 (19 pax), the elevator has an area of 19.3 sqft. Using the logic above, this surface when deflected 5 degrees at cruise speed would feel a force of approximately 421 lbf. Your actuator sizing will ultimately need to account for the parameters above (and more), but hopefully this is an informative starting point.

There are two things missing: The control surface is part of an empennage which carries a load even without control surface deflections. Next, the question should be more about the hinge moment on that surface - that, divided by control horn length, is what determines actuator loads.

@peterkampf I see. When designing the control mechanisms, wouldn't the the hinge moment be a design target? In other words, wouldn't you tweak the control horn and hinge locations to obtain an adequate hinge moment, where "adequate" is driven by the amount of force the control surface must oppose when deflected?

Yes, if you add area ahead of the hinge, loads go down. The best mechanism I have personally seen was on a Canberra - its ailerons could be moved with very low forces at flight speeds up to Mach 0.8. The downside here is an increased flutter risk - such mechanisms were the result of desperation.

Sin5°=-0.9589,sin 25=-0.1324,As angle of deflection increases,exposed area(Asin sigma)should increase not decrease......do you think this invalidates the above formula(the 3rd one)?


text:
How does an increase in climb rate affect climb gradient?
I'm learning to take (and pass I hope ;)) my PPL exam and I've just found one question, that I'm not 100% sure about.
How is climb gradient going to change (for small angles) while rate of climb increases 2 times? (TAS = constant)
a) stay the same
b) increase 2 times
c) increase 4 times
d) decrease 2 times
Can I calculate climb gradient as (rate of climb/TAS)? If yes, the correct answer is B.
Guys, could you please confirm?

For small angles, climb gradient can indeed be approximated using ROC/TAS. Easiest if you draw a sketch for things like this!


text:
How do I calculate true altitude?
Currently doing my CPL and in class we were shown a formula on how to calculate the true altitude, which as I understand it is the distance between the ground and the plane while taking into account non-standard temperature. Basically, what a radar altimeter would read back to you.
The formula I was given for this lesson was the following:
$$
\begin{equation*}
\frac{\tiny{(Indicated~Altitude-Airport~Elevation})}{1000}*\small{(STD~Temp.~at~Indicated~Altitude-Actual~Outside~Temp.)*4}
\end{equation*}
$$
I have a hard time wrapping my head around the logic of this equation. For instance, why is it that we would need the airport elevation to figure out how high we are relative to the ground below us?
Also, why is it that we can't simply use the part in the density altitude formula that derives the altitude change incurred because of the difference between OAT and standard temperature at a given altitude?
Let's take the following problem:

Airport Elevation:   500ft MSL
OAT:                  -10c
Mountain Height:    2000ft MSL
Indicated Altitude: 2500ft
QNH:                 29.42
Given the following information, will the airplane clear the
  mountains?

The way I would personally calculate it is by converting indicated altitude into pressure altitude, then calculating the difference in temperature between OAT and standard temp. at pressure altitude, then multiplying that number by 117 (as you do to find density altitude). In that case the equation would be as follows:
$$
\begin{equation}
Pressure~Altitude=(29.92-29.42)*1000+2500\\
Pressure~Altitude=3000ft\\
\Delta T=(15-\frac{2}{1000}3000)--10\\
\Delta T=19\\
True~Altitude=2500ft-\Delta T*117\\
True~Altitude=2500ft-19*117\\
True~Altitude=277ft
\end{equation}
$$
This answer however, turns out to be wrong according to my instructor though he was not able to give me an explanation on why that was beyond saying that I shouldn't think too much about it and just apply the formula that was given. That formula gives a true altitude of 2348ft for the plane, meaning that it will clear the mountains by about 300ft.

True altitude is the height above sea level, not the height above the ground. What you are describing is "Absolute Altitude" https://en.wikipedia.org/wiki/Altitude


text:
What are "referred" speeds and "referred TET"?
i'm currently reading the Rolls Royce publication The Jet Engine. It makes reference to "referred speed", "referred TET" and "referred mass flow". Referred speed seems to be defined as N/sqrt(T). I guess that N is engine speed but I'm not clear on why it is divided by the root of temperature. Also can someone advise how referred mass flow and referred TET are defined please?

Welcome to aviation.SE! I added a link to the book you mentioned, if it's wrong then you can just edit again to correct it.

Yes that is the correct book.

Referred speed is a way of compensating for the fact that air density changes with temperature.  For example, let's say you are designing a new turbojet engine. You fire up your engine one warm sunny day in July, and note that when the shaft speed is 5,000 rpm, it's making 10,000 pounds of thrust.  But then later, on a cold winter evening in December, you run the engine to 5,000 rpm, and now it's making 11,000 pounds of thrust!  What happened?  Did your engine suddenly get more efficient?  No.  It's just that colder air is more dense.  So for that same 5,000 rpm, you are pumping a lot more mass of air. Now, where referred speed comes in.  You are going to have to tell the pilot of the plane how to set thrust.  i.e. he wants to get 10,000 pounds of thrust, no more and no less.  You can't just tell him to set 5,000 rpm, because that will be a different amount of thrust depending on the ambient temperature.  So you tell him to set a specific "referred speed".  If you go to the same referred speed, you'll always get the same amount of thrust, no matter what the temperature.
You should also look up corrected speeds.  That is a very similar concept.

Good simple answer. I would suggest corrected and referred speeds are basically exactly the same thing, except maybe the correction factor in the formula for referred speeds is specific to a certain engine type, where was the corrected speed formula is assumed to be the same for any gas turbine.


text:
Is the flight management system software independent from the hardware?
Is it possible to 'mix and match' hardware and software from different vendors to make a flight management system? I want to prepare a study about FMSes and I can see that there are several independent software providers, does that mean their software can be installed on different hardware platforms?

Hardware with software?

I edited your question a lot to say what I think you're asking, but if I got it wrong please just reverse the edit, or edit it more yourself.

Thank you @Pondlife for your help. I want to know whats the use of  merlot.aero and aims.aero within flight management system.

I looked at those web pages and it looks like they're operations management tools, not on-board avionics or flight management systems. A "flight management system" (FMS) is something very specific.

The flight management system (FMS/FMC/MCDU) is both hardware and software. The hardware is the structure of the computer itself, while the software is the actual software that is powering and enabling user input into the computer.
Think of it like your computer.

This does only partially answer the question, probably since the question has been edited. The OP asks (apparently) whether hard- and software can be combined from different manufacturers.

Thank you all for helping me out. I want to know what is the use of https://www.merlot.aero/ and http://www.aims.aero/ within flight management system

If you are thinking of the analog of installing windows or linux on a given physical personal computer then the answer is more or less no. Not because its not possible but more because thats not how avionics are made or how the avionics pipeline has developed over the years. Avionics also work quite differently than your basic home computer as they are designed to be far more redundant and have far more real time conscious operations. You, as a user a free to run any software you like on your computer as its unregulated and does not matter if say one process blocks another, a process fails, a process produces erroneous outputs or a process all together destroys the machine, these things would not be great in avionics.

There are two basic flavors of avionics "software" components/updates:
Database updates:  these occur regularly when a new dataset is made available and update the waypoints, VOR's, Frequencies, and Maps in the system.
Database updates all ultimately contain the same information (generally speaking). Depending on where you are in the world the information is maintained by various regulatory authorities as well as some other companies. However to play nice with a given FMS the data may need to be delivered in a specific format. As far as I know there are no public FMS data formats and generally you need to get the database updates directly from the maker of the unit.
These could be interchangeable with some coordination but the various makers have a steady income stream from the updates so there is little incentive to change the model.
Data sets from someone other than the manufacturer will most likely not load and may come on the wrong card type or be incompatible with the loading system. Likewise not all units are capable of consuming/making use of the full data set.
Software Updates: these occur perhaps on a schedule or perhaps when needed and change the functionality of the system. Or if there is a bug in the system.
Avionics tend to be quite proprietary largely because it costs a lot of money to certify and design avionics on top of this manufacturers of avionics components have certain responsibilities to upholding certificates. If problems arise with the unit arise they may need to provide a fix, as such its simpler to have control over the whole unit, both soft and hardware. This also helps to cut down on the potential combinations of software/hardware units in the event of an issue either on the software or hardware side, it also absolves you from needing to certify all possible combinations.
One things thats very important is that due to the use case, avionics tend to blur the lines between hardware and software in terms of what is implemented where. Keep in mind that generally anything that can be implemented in code can be implemented in pure discrete logic. Similarly most things implemented classically in hardware can be modeled in software. As such the blurred lines and often tight coupling and required knowledge across the platform makes it almost required to do it all as one.
You can find a lot more about the FAA's hardware and software certification process here as well as some notes on avionics software here. There is also some decent tangentially related info in this answer.
This group seems to be building some kind of open source airplane with open source avionics and might be worth reading up on to see where they are at and how they went about doing it.
But its not all for naught, things like the ARINC standards have been developed allowing for the creation of Integrated Modular Avionics and pushed for standardizing at least parts of the equation and opening future possibilities of at least mixing and matching components.

Thank you all for helping me out. I want to know what is the use of merlot.aero and aims.aero within flight management system


text:
Can I fly a UAS that weighs more than 55lbs if I'm on private property?
Can I fly a UAS that weighs more than 55 pounds within my own private property if I stay below say, 200 feet? I do not have 333 exemption

It may help to list (generally) where you are. Airspace regulations extend to the surface and the type of airspace you are in may effect the answer.

Even on your own property, a UAV can lose the control signal, or malfunction, and travel well outside of your property before impacting heaven knows what. The larger the UAV, the more damage from the impact, the propellers, or the resulting fire if the liquid fuel ignites or the li-ion batteries short circuit and ignite. I believe that's the FAA's concern over large UAV's... not like a hobby quad copter that will just bounce off of a building or a car, and not like a land based vehicle that usually won't go far before impacting something in an uncontrolled situation.

The FAA regulates the airspace from the surface to 60,000 feet regardless who owns the land property beneath it.

TL;DR
According to the FAA Part 333 rules, all UA aircraft (> 55 lbs) must register with the FAA with an N-number before flight. As far as I can tell, the rules don't specify zones or altitudes where the regulations aren't in effect, so I think it is highly advisable to register.
My personal rationale
Being safe just in case is far better, at least in my mind, than choosing to avoid registration. I know people in my RC Quadcopter possee that fly sUAS aircraft but don't have Part 107 licenses. I have one, (I would rather be safe than sorry) but I could see how as long as you are careful, flying a small sUAS without a Part 107 license isn't such a terrible option.
In my mind, UA aircraft are an entirely different story, as they are inherently significantly larger than sUAS aircraft, and if something does go wrong, damages would be more severe. The risks (not to mention the responsibility) involved are far more substantial, and I would never consider flying a UA aircraft without a license even if I was only flying on my own property.

I think you mean "never consider flying a UA aircraft wit no liscence" at the end, but didn't want to suggest edit for something potentially involving meaning.

@starweaver yes. Thanks for the suggestion!

Flying over your own property doesn't matter.  As soon as you leave the blade of grass, whether at 1 inch or 1 mile, you are under FAA domain.
Therefore in your case, you would need to comply with appropriate regulations.
If you have a special application, or you are doing some specialized development, you might look in to a waiver.  But in general, expect that all the rules apply to you even if you are over your property.  This includes prohibitions about flying over people, night operations, etc.

No one considers "flying while touching the ground somehow" to be flying, so the opening soulds contradictory.

@StarWeaver, good point on English usage, and I have edited the answer per your point.

IDK about that 1 inch.  I mean, if I imported a Flymo https://en.wikipedia.org/wiki/Flymo would I have to register it with the FAA?

@jamesqf: The Flymo (my dad had one) is connected to the ground by the pilot, if that makes any difference. Now a ride-on Flymo ... Do hovercraft fall under FAA in the USA? I was intrigued on a jetfoil journey between Ostende, Belgium and Dover, Britain, many years ago to have a pre "take-off" briefing by the cabin crew.

@Transistor: Well, how about a remote-controlled Flymo, then?  Or one that's purely robotic?

Sure.  In the eyes of the FAA, a UAS over 55lbs is no different from any other aircraft operating in the national airspace system and subject to the regulations under Title 14 CFR.


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Are pilots over the age of 65 allowed to act as PIC for ferry flights between the UK and Canada?
I have been informed that I'm not allowed to act as PIC for a ferry flight to and from the UK for a Canadian commercially-registered transport aircraft that I currently fly in Canada for charter ops.
Does anybody have experience with this scenario and can provide input?
I have been told it is only permitted for privately-registered aircraft. However, My current understanding is that the regulations would apply internationally based on the national authority that the aircraft is registered under.

Welcome to aviation.SE! See this question, it looks like ICAO limits international commercial operations to 60, or 65 if you have a younger pilot on board too.

For what it's worth I agree with your understanding. Australia doesn't have a mandatory pilot retirement age, and our pilots over that age are allowed to fly into countries with such a restriction, as long as they are on an Australian registered aircraft. But I have nothing to confirm that the same applies for your situation.

@Pondlife Note that this applies to "commercial air transport" operations, not just commercial operations (the question that you linked to specifically asks about Part 135).

I cannot answer for Canada, but I would guess that it is similar to US rules (most of them are), and the FAA has a rule which says that when you are flying outside of the US that you must comply with FAA rules AND with the laws in the country in which you are flying, as well as some of the ICAO guidance.  That being said, a ferry flight isn't typically a flight that falls under air carrier rules (in the US), so may be allowed.  I'll have to let someone with actual knowledge of Canadian rules provide an official answer though.

Ok thanks for input, I will be consulting with an aviation lawyer in the next few days who specializes in ICAO law and its application to Canada and other states.

@AlanVittery Please come back and let us know what the lawyer said!

@Lnafziger Good point! Hopefully it at least gives some context and helps the OP to ask the right questions with a lawyer.

Canada has no max age for any pilot, in any type of operation, in Canadian airspace.  So the question becomes “What is the max age for a pilot operating a Ferry flight of a commercial aircraft in the UK?”


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Are there standard visual cues or other methods for flaring in a B737-800?
Just a newbie in the 737. I've flown 100 hours - in real life - in this lovely beast and found the landing to be very confusing: my main problem is every time just from 50 feet, the sink feels too much. I don’t understand the right time to flare, almost every time it’s early or late. Also that I can’t flare at one go. Very very frustrating. Maybe a lot to do with seating position? Can you suggest the right visual cues or other techniques to use?

Are you talking about sim or real-life?

Real life brother

One thing that might help you is putting the arm rests at maximum height. This allows you to control the yoke more precisely, by resting your arms firmly on the arm rests.

Have a look at this by James Albright: https://code7700.com/landing_flare.htm
He suggests two things.

A "scientific" flare ought to be possible, i.e. an ideal flare should take a certain number of seconds. I'm not sure what this number is in a B737. We are given a number for rotation: it should take approx 7 seconds. Here https://www.code7700.com/landing.htm he says a flare should take no more than 4 seconds.
Keep a landing diary! Describe your landings in writing, and you will find it easier to improve them.

In my airline, we are told to start to flare the 737 at 20 feet. With a descent rate of approx 10 feet per second, I start thinking about flaring at 25 feet, to give my brain half a second to get ready. The throttle should be closed smoothly, not suddenly.

I'm not sure I understand what you're asking, but proceeding from what I think you may be asking, consider the following.
Are you flaring from a stabilized approach? Until a pilot internalizes things for any large aircraft, using a stabilized approach on speed appropriate to your landing weight gives a common, starting reference to work from.
From such an approach you should have been given in training the altitude from which to start the flare. For purposes of discussion, let's say that's the 50 feet you mentioned. Thus, the first problem is determining when you're at that point. Assuming you have a radar altimeter, are you using the call outs from the altimeter to consistently start the flare at the same altitude?
Now, in a stabilized approach, you don't feel a change in sink rate in the seat of your pants until you actually start the flare, so when you say "the sink feels too much", I'm assuming that you're having trouble with the rate of your flare. In other words, how much back pressure you should be exerting on the yoke. My thought would be that you could try focusing on how much to raise the nose at the 50 foot call rather than trying for a given amount of feel in the seat of your pants. I never flew the 737, but on the 747, at the 50 foot call, I would would raise the nose to 5° higher than it was during the stabilized approach. Then at the 10 foot call, I would raise it 2° higher yet. There would, of course, be an appropriate power reduction each time.
The 737 will have different numbers than the 747, but what I'm suggesting is to try doing it by the numbers, consistently, every time. Any 737 instructor should be able to tell you what those numbers are. You might even try posting a question here asking specifically what those numbers are. Perhaps a 737 pilot will see this answer and provide them.
Finally, an advantage of doing things by the numbers is that it really doesn't matter what your seat position is, although you should certainly go for a seat position that will eventually allow you to best internalize what the numbers mean insofar as the view out the windshield. When I transitioned from the F-27 to the SA-227, I found the view out the front confusing, but no matter since they taught us to simply do it by the numbers at first.
My apologies if my answer has been too pedantic or if I have misunderstood the question. If so, let me know in a comment and I'll be happy to delete the answer.

Sir, at first take my greetings. Wish I had met such a humble person in real life. Yes sir my problem is how much to pull confusion. Even if I come with a beautiful approach in calm wind, when I shift my view to the tress on the end of the runway, I feel like I am falling fast. And things get rough. Sometimes I try flaring at 30, 20 ; and then cut power at 10. Works and sometimes doesn’t. If only I could take a reference.  Thanks again Sir.

And sir that you mentioned about pitching up 5 degrees. Sir do you look at instruments and pitch up?

@AsifNizam Do you really need to "shift my view to the tress on the end of the runway."  If you're on the glidepath and aligned with the runway, you know you're not going to hit the trees, so why look directly at them. My thought would be that it's better to be looking at the VASI/PAPI and the runway touchdown zone and then down the runway. Just a thought.

@AsifNizam Yes, we did use the artificial horizon for pitch attitude in the flare to get the initial 5 degrees nose up increase. Some would say you shouldn't have your eyes in the cockpit at all during the flare, but it worked for us. In the 747, though, it was easy as it came down the glideslope at 5 degrees up, so adding another 5 put it at 10 degrees and there was a 10 degrees mark on artificial horizon. That made it easy to see with a quick glance. The additional 2 degrees at the 10 foot call we mostly did with just a bit more yoke movement. – Terry 8 mins ago

Thanks a ton Sir. I hope I can scan the attitude indicator this time. Will sure let u know if this works. :)


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Does the FAA restrict the color of pilot seat belts?
In the US, is there a legal restriction to prevent the use of bright red pilot seat belts in GA aircraft?

What jurisdiction? What type of aircraft? I've not seen bright red color seat belts in the US GA aircraft I have been in.

For GA US registered aircrafts

The only red seat belts I remember seeing are in my Cherokee, but the rest of the interior is red, too.

My concern is possible visual interference with emergency buttons or levers that must be easily found, but I did not get any FAR reference that address this.

This company rewebs seat belts and has a whole bunch of colors, including red, so I’d so no. http://aircraftseatbelts.com/aircraft-seat-belt-colors/

This company also sells red belts. http://www.cmaviationservices.com/aircraft-seat-belts/

Is there a particular reason for this question? Did someone tell you there was such a regulation?

No, there's no regulation on the color; the restraints just have to allow the crew to perform all their duties.
The old 23.785 regulation said:

(e) The restraint system for each crewmember must allow the crewmember, when seated with the safety belt and shoulder harness fastened, to perform all functions necessary for flight operations.

The new 23.2535 regulation says even less:

Safety and survival equipment, required by the operating rules of this chapter, must be reliable, readily accessible, easily identifiable, and clearly marked to identify its method of operation.

And empirically, there are many colors available. Googling will bring up plenty of images.


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Are electronic copies of aircraft logbooks acceptable for reconstruction?
This question about a pilot losing the logbook for an aircraft got me thinking about acceptable means to reconstruct logbooks. Reconstructing the logbooks is not a trivial process (or a cheap one). Aircraft logbooks can be lost, misplaced, or destroyed for many different reasons.
There is a big focus on electronic flight bags (EFB's) with electronic logbooks, some sites dedicated to providing electronic logbook services for pilots, but nothing about the airframe/engine logs themselves.
So the question is, would the FAA and potential buyers accept electronic copies of the airframe/engine logbooks as legal record equal in value to the paper copies?
Edit
I'm curious if digitizing old records (scanning) is acceptable for electronic records. The AC for digital records has a lot of emphasis on digital signing by the original signatory, but isn't so clear on the use of a scanned copy of a signature being an acceptable backup for paper records.

According to FAA's advisory AC 120-78A, it is perfectly acceptable to maintain electronic records for airframe/engine.

I'm going to edit the question but I'm also interested from the point of view of digitizing old records. AC 120-78A puts a lot of emphasis on digital signing, I'm curious if I scanned the logbooks if this would qualify under those considerations.

It seems that in true FAA fashion, there are no concrete details on what exactly is allowed. As it is an application process, where one must detail/demonstrate how they plan to implement the transfer to electronic records, it is very much a case by case basis for approval. It is entirely possible that two different applications, both planning to scan logs, could have different outcomes based on how those scans were going to be managed after the fact.

@RonBeyer Per that AC you would need to demonstrate that the content could not be modified after signature if you're keeping a purely electronic log. For scans that's a bit difficult (you could photoshop the logbook entry). The FAA does seem to consider photocopies of a logbook acceptable for reconstruction if the originals are destroyed though, so on that basis printing copies of the scanned logs would seem to be kosher. Whether they're "equal in value" to the originals is a matter between you and any buyer though: The FAA just cares that you have the required records documented.

Advisory Circular 120-78A (pdf), “Electronic Signatures, Electronic Recordkeeping, and Electronic Manuals,” gives FAA guidance on the use of electronic logbooks. Summary, with added emphasis:

This advisory circular (AC), as any AC, is not mandatory and does not constitute a regulation; rather, it provides standards and guidance for electronic signatures, electronic recordkeeping, and electronic manual systems. Electronic recordkeeping systems/programs are used to generate many types of records (e.g., load manifests, dispatch release, aircraft maintenance records, maintenance task cards, pilot training records, flight release, and/or airworthiness release). This AC describes an acceptable means, but not the only means, for a certificate holder to utilize an electronic signature, electronic recordkeeping, and electronic manual systems.

This guidance does allow for certain certificate holders to proceed in transferring records and even digital images of hardcopy signatures to electronic recordkeeping systems. Read on for details.
Acceptable electronic signatures
Electronic signatures have many acceptable forms, with examples given in 2-1b.

b. Types of Electronic Signatures. Electronic signatures may appear in various formats. No matter the format, they must meet the legal requirements of electronic signing that appear in subparagraph 2-1c. Examples of electronic signature formats include, but are not limited to:

A digitized image of a handwritten signature that is attached to an electronic record;
An electronic code (e.g., a secret code, password, or personal identification number (PIN)) used by a person to sign the electronic record;
A unique biometrics-based identifier, such as a fingerprint, voice print, or a retinal scan; or
A digital signature.

Transfer of records and signatures
Digitizing or scanning paper logs in terms of the AC is transferring data from a legacy system, covered in 3-3b.8 and 3-3b.9.

(8) Transferring Data. Technological advances may make it desirable or necessary for a certificate holder to update its electronic recordkeeping system or transfer data to a new system. The certificate holder must have policies and procedures that ensure the continued integrity of record data when a certificate holder moves records from one system to another. This could entail running redundant systems for a brief period of time.
(9) Continuity of Data Between Legacy and Electronic Systems. The system should have a method of ensuring continuity of data during transition from a legacy (hardcopy) system to an electronic system.

Moving ahead
Subparagraph 1-8e gives broad latitude on moving forward.

e. There is No Requirement for Formal Approval, Acceptance, or Authorization for Part 61, 63, 65, 91 (Excluding 91K), 137, or 183. The use of an electronic signature, electronic recordkeeping system, or electronic manual system under part 61, 63, 65, 91 (excluding 91K), 137, or 183 does not require formal FAA approval, acceptance, or authorization. OpSpec (including an MSpec/TSpec/LOA) A025 does not apply to operations under these parts. The FAA recommends that all certificate holders adopt the standards for electronic signatures, records, and manuals as set forth in the AC, regardless of whether or not approval, acceptance, or authorization is required. If a required signature, record, or manual is provided in an electronic format or application that does not meet the standards set forth in this AC, the FAA may question its validity. If the FAA determines that an electronic signature, record, or manual does not meet the standards or is otherwise unacceptable, the FAA office with oversight responsibility will notify the certificate holder in writing. Upon receiving notification, it is incumbent upon the certificate holder to make the appropriate corrections.

Complete electronic records
Chapter 3, “Electronic Records,” details the elements that should or must be in an electronic aircraft logbook, pilot logbook, etc.

3-1. ELECTRONIC RECORDS. An electronic record must provide equivalent or better data integrity, accuracy, and accessibility to what would otherwise be provided by a paper record. In general, a record preserves the evidence of an event. It should contain enough information to clearly depict the event that took place. It is the certificate holder’s responsibility to address all 14 CFR requirements for their recordkeeping system(s) applicable to their operation(s).
3-2. FAA STANDARDS FOR ELECTRONIC RECORDS. To be considered complete and
  valid, an electronic record should contain at least the following information:

The type of event that took place (e.g., training, maintenance performed, signing of a release, conduct of a flight, etc.);
For a training event, information that shows compliance with regulatory requirements, such as the name of the course module or subject, the number of hours of instruction, whether the student passed or failed, etc.;
When the event took place (e.g., the date and time (where appropriate));
Where the event took place (e.g., the station, training facility, maintenance facility, etc.);
Who was involved in the event (e.g., crewmember, dispatcher, instructor,
  mechanic, etc.);
Aircraft type and registration number for pilot logbook records (when required by regulation);
Certification, verification, or authentication of the event, such as a signature, where required by regulation; and
Applicable aircraft, airframe, engine, propeller, appliance, component, or part make and model (M/M) for maintenance records, such as life-limited parts and time-in-service records.

Other guidance
AC 120-78A gives additional guidance on elements that electronic recordkeeping systems should provide.

Security

Protect confidential information
Ensure that information is not altered in an unauthorized way
Secure access and safeguard against unauthorized access

Procedures

Making required records available to FAA and NTSB personnel
Quality Control
Maintenance Support and Backup Measures
Record Transfer
Persons with Authorized Access
Electronic Authentication, Signature, Validation, or Endorsement
Training and User Instructions
Transferring Data
Continuity of Data Between Legacy and Electronic Systems
Continuity of Records for Maintenance Providers

Responsible Personnel
Description of Electronic Recordkeeping System(s)

Facilities, hardware, software
Identify records to be maintained
Identify records subject to electronic signature

Changes to the Electronic Recordkeeping System
Audit Procedures

AC 120-78A Table of Contents

General Information
Electronic Signatures
Electronic Recordkeeping
Electronic Manual Systems
Voluntary Discontinuance by the Certificate Holder
Administrative

Appendix: Sample Letter of Intent


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Why do most fighter craft have a center stick?
Is there a specific reason why in most fighter planes, the control column is between the pilots legs instead of on the sides like Airbus airliners? I know the F-16 has sidesticks, but most of the others have it in the middle.

F-16 Cockpit with sidestick

F-15 with centerstick
Images from Wikipedia

Possible duplicate of What are design or functional differences between sticks and yokes?

Related, but doesn't provide the answer: Centre stick (a voluntary to contribute to Wikipedia if the answer is given here?)

@Simon Nothing related to what I am asking

Check when they were built. Most of the current generation seem to be side stick, contrary to what you claim in your question.

I remember something once about why a side stick was chosen for some fighter jet design (I forget which one in relation to this statement) - during high-G maneuvers, the arm-rest would help support the pilots arm.

Back in the day of mechanical controls, a center stick would have been easier to build -- better leverage for the pilot.

Read the answers to that question.  The reasons for side stick vs centre stick are clearly given.

The F-22 and F-35 also have side sticks

Shows the F-16 photo really the stick used in the Viper? It looks very much like my Thrustmaster HOTAS Warthog Flightstick^^

@jklingler, maybe your Warthog Flightstick looks very much like the F16's stick, since it probably came first.  ;)

@FreeMan Negative, the warthog flight stick is a replica of the A10C's stick. I doubt that the F16 uses the same.

At present, it appears to be a design choice rather than any other thing.
Most of the present generation fighters are transitioning to side-mounted sticks from center sticks. Almost all of the US combat aircraft have side-sticks. In Europe, it's somewhat of a mix- Typhoon (and Gripen) have center sticks, while Rafale has a side stick. The Russians are sticking to a center stick in the T-50.
The location of the control stick depends on a number of factors:

Ergonomics- Side stick is better in this regard, limiting the display area in the center; this might be a problem in smaller cockpits.
One of the reasons for using side stick in F-16 was that it (the hand-rest) supported the Pilot's hand during high 'g' maneuvers, along with the inclined seat (which is also being used in a number of aircraft).
There is also the personal choice- some pilots may prefer using a center stick, and others, the other way around (there seems to be little problem in transitioning from one to another).

Historically, combat aircraft have had center sticks. Side sticks have come about after fly-by-wire controls. Eurofighter has the following to say in this regard:

Every military aircraft cockpit design has to be something of a compromise, but in a small cockpit, a center stick can cause particular ergonomic difficulties, perhaps limiting the size of the display area for the instruments and/or screens ... That is when options are considered, and one of those would be the sidestick as used in the F-16. However, if the cockpit is sufficiently large and the geometry of the seating position is correct, there is no reason to have anything other than a center stick—and the pilots of the four partner nations agreed. The choice between center sticks and sidesticks is 40% personal—the balance being on technical grounds. We find that F-16 pilots convert easily and naturally to a center stick.

It would be interesting to hear what the "technical grounds" for preferring center sticks are. As a layman without any particular relevant experience, I find it difficult to imagine them by myself.

A complete guess, but center stick allows for a longer stick and therefore more force. Could be important when you don't have any kind of hydraulics.

@Masse Please note that side-sitck is found in ony fly by wire control aircraft. While you may be correct that in small GA aircraft longer stick equals more force, in modern combat aircraft like Eurofighter Typhoon, the size and movement of the center stick is no more or less than the sidestick.

I assumed the center stick was historical, meaning that when the first (fighter?) aircraft were developed, the stick was located in between the pilot's legs because that was the best place that all the control linkages could be run to. Is that not a factor at all?

@ToddWilcox I guess it comes under the 'personal' column. :) However, when you're trying to squeeze every drop of performance from your aircraft and pilot to avoid getting killed, history is of little value. Also, with fly-by-wire, it doesn't matter where the stick is.


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How do large drones communicate with ATC?
For large drones like the Reaper, X-47, and aircraft like that, how do the "pilots" communicate with ATC when they need to?

they don't the guy flying it does

@ratchetfreak: By phone using only ground channels from pilot location to ATC? Or do they use the done radio?

My guess is they don't. They're usually used over foreign soil where the whole idea is that nobody knows they're there. If they're flying around the home country for training or something they probably stay within an active MOA. I don't think they're flying them around cross-country. Although it would be interesting to see one landing at LAX.

Related

"On 18 September 2014, the Triton successfully conducted an 11-hour cross-country flight from Northrop Grumman's Palmdale, California facility to Naval Air Station Patuxent River in Maryland. ... The aircraft flew a pre-approved instrument route along the southern U.S. border" - https://en.wikipedia.org/wiki/Northrop_Grumman_MQ-4C_Triton#Flight_testing

All drones independent of size assuming they aren't model aircraft would have to have some means of contacting ATC such as wandering into controlled airspace.

They have an onboard VHF radio that can be used to receive and transmit on local frequencies.
For example, on the MQ-1B Predator you will see a small fin on the back of the aircraft behind the GPS dome. That is an omni-directional antenna which can transmit VHF. The Global Hawk has a similar capability.